Optical grating and method for the manufacture of such an optical grating

ABSTRACT

An optical grating has a multiplicity of parallel diffraction structures, which are arranged on a support defining a base face. Each structure has a blaze flank that is inclined substantially at the Littrow angle with respect to the base face, and a back flank. Both flanks together form a reflection layer which comprises a reflective base layer and a transparent protective layer that is connected to the base layer and covers it. The protective layer on the blaze flank and the protective layer on the back flank are made of the same material. The thicknesses of the protective layers on the blaze flank and on the back flank, however, are different.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/885,411,filed Jul. 7, 2004, which is a continuation of International ApplicationPCT/EP03/00018, with an international filing date of Jan. 3, 2003, whichwas published under PCT Article 21(2) in German. The full disclosures ofthese related applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical gratings and to opticalarrangements comprising such gratings. The invention furthermore relatesto a method for manufacturing such optical gratings.

2. Description of Related Art

Optical gratings are required to have an optimized performance for manyoptical applications. To date, optimal performance was synonymous withmaximization of the reflection efficiency since, according to theclassical approach, this also entails minimization of the absorptionlosses of the incident light in the grating. An optimized reflectionefficiency makes it possible to achieve an intended output light powerwith a low input light power, which reduces the demands on the lightsource. Minimizing the absorption in the optical grating leads to animprovement in the long-term stability of the optical grating, since thethermal load on it due to the incident light power is reduced.

SUMMARY OF THE INVENTION

It is a first object of the invention to increase the performance ofoptical gratings and optical arrangements containing such gratings.

An optical grating according to the invention has a multiplicity ofparallel diffraction structures, which are arranged on a supportdefining a base face. Each structure has a blaze flank that is inclinedsubstantially at the Littrow angle with respect to the base face, and aback flank. Both flanks together form a reflection layer which comprisesa reflective base layer and a transparent protective layer that isconnected to the base layer and covers it. The protective layer on theblaze flank and the protective layer on the back flank are made of thesame material. The thicknesses of the protective layers on the blazeflank and on the back flank, however, are different.

The new grating of the optical arrangement has protective layers with anaverage layer thickness which, even though layer thicknesses will oftenbe referred to below without being more specific, do not always need tobe the same as the local layer thickness. In the case of gratings with asmall reflective area, the layer thickness of the protective layers isvirtually constant everywhere on this area, so that the average layerthickness is equal to the local layer thickness. In the case of gratingswith a larger reflective area, there may be a variation in the layerthickness so that the average layer thickness differs from the locallayer thickness. Nevertheless, since the optical effects of layerthickness variations are generally averaged out when the area of thegrating is considered as a whole, the definition of an average layerthickness will generally provide optimized performance of the gratingoverall.

The invention is associated with two surprising discoveries, which wereobtained by optical calculations using electromagnetic diffractiontheory:

On the one hand, the performance of an optical grating depends on thelayer thickness of the protective layer on the back flank. Since theback flank is not generally exposed to the light from the light sourcewhen the optical arrangement is being used in Littrow operation, such afinding does not agree with classical geometrical optics. As has beendiscovered with the aid of electromagnetic diffraction theory, it isnecessary to take into account an additional interaction which is due tothe fact that some of the incident light propagates very close to atleast one section of the back flank. This interaction can be compensatedfor by suitably dimensioning the layer thickness of the back flank so asto minimize the absorption of the incident light. Owing to a higherbreakdown threshold, minimizing the absorption leads to greaterdurability and a very stable optical grating, in particular one withoutthermal drift effects.

The second surprising discovery which is provided by the opticalcalculations is that the layer thicknesses of the protective layer, forwhich the absorption is minimized, differ from those that maximize thereflection efficiency, which is a second quantity affecting theperformance of the grating. Therefore it is not possible to employreflection measurements when optimizing the layer thickness of theprotective layer on the back flank in relation to absorption. Rather,the absorption optimization must be carried out independently of thereflection optimization. As has been discovered, optimum, i.e.minimized, absorption does not therefore automatically imply optimum,i.e. maximized, reflection efficiency, because even if the absorption isminimal, reflected components of the incident radiation may also befound in diffraction orders other than the useful one, which do notcontribute to the reflection efficiency.

Commercial design programs for the layer optimization of thin-filmsystems are available for calculating the layer thicknesses. Anoptimized grating can thus be modelled with relatively little expense.However, it is also possible to calculate a layer thickness of the backflank as an empirical approximation even without using electromagneticdiffraction theory.

In many cases, a multilayer system as protective layer on the blazeflank leads to a further improved reflection efficiency, as is known forthe coating of flat substrates.

With light having a TM polarization when it impinges on the grating, aparticularly advantageous effect on the grating performance is obtained.

In the ideal case, the phase-dependent optical performance of atransparent optical layer operated in double transmission is repeatedwhen its optical thickness is increased by an integer multiple of halfthe wavelength. An optical grating, in which the protective layer on theback flank has an average layer thickness which is larger than theaverage layer thickness of the protective layer on the blaze flank, isgenerally obtained when the associated minimum layer thicknesses neededin order to achieve optimized performance are defined for the layerthicknesses on the blaze flank and on the back flank. This relationtherefore represents a first approximation for the production of anoptical grating with increased performance.

The smaller the absolute layer thickness is, in general, the moreaccurately these layer thicknesses can be produced with known coatingmethods. An optical grating with layer thicknesses less than 100 nmtherefore leads to a grating with increased performance, which isrelatively simply to produce in a reproducible way.

An optical grating in which the base layer has a metal surface can beproduced such that it has a relatively good performance even without anyapplied protective layer, and it therefore has good prerequisites foroptimization by applications of the protective layer.

In the case of an optical grating in which the metal surface is formedby a metal layer applied to the support, the support material may beselected irrespective of its reflection properties so that, for example,the mechanical or thermal properties of the grating can be improved bythe support material. An aluminum surface has a high reflectivity.

A dielectric layer is particularly suitable for the production of aprotective layer for a grating optimized with respect to itsperformance.

It is also an object of the present invention to provide a method forthe manufacture of an optical grating in which a predeterminedlayer-thickness relation between the layer thicknesses on the blazeflank and on the back flank can be achieved with the greatest possibleprecision.

According to the new method, the blaze flank and the back flank arecoated simultaneously. This shortens the production compared with amethod in which the blaze flank and the back flank are coated separatelyof each other. Furthermore this eliminates sources of error which couldlead to a deviation from the predetermined layer thickness ratio, owingto differences between sequentially performed coating steps.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention will be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 shows a section through a detail of a Littrow grating, thesection plane being perpendicular to the direction in which thediffraction structures of the Littrow grating extend;

FIG. 2 shows a similar section to FIG. 1 through a support for a Littrowgrating according to FIG. 1;

FIG. 3 shows the dependency of the reflection efficiency of a Littrowgrating according to FIG. 1 on the layer thicknesses C_(b), C_(l) on theblaze flank and on the back flank of the Littrow grating, in athree-dimensional representation;

FIG. 4 shows the dependency of the absorption by a Littrow gratingaccording to FIG. 3 on the layer thicknesses C_(b), C_(l) on the blazeflank and on the back flank of the Littrow grating, in athree-dimensional representation;

FIG. 5 shows the dependency of the reflection efficiency of analternatively coated Littrow grating according to FIG. 1 on the layerthicknesses C_(b), C_(l) on the blaze flank and on the back flank of theLittrow grating, in a three-dimensional representation;

FIG. 6 shows the dependency of the absorption by a Littrow gratingcoated according to FIG. 5 on the layer thicknesses C_(b), C_(l) on theblaze flank and on the back flank of the Littrow grating, in athree-dimensional representation;

FIG. 7 shows the dependency of the reflection efficiency of anotheralternatively coated Littrow grating according to FIG. 1 on the layerthicknesses C_(b), C_(l) on the blaze flank and on the back flank of theLittrow grating, in a three-dimensional representation;

FIG. 8 shows the dependency of the absorption by a Littrow gratingcoated according to FIG. 7 on the layer thicknesses C_(b), C_(l) on theblaze flank and on the back flank of the Littrow grating, in athree-dimensional representation; and

FIG. 9 shows a section through a detail of a Littrow grating which iscoated with a multilayer coating system.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a detail of a Littrow grating denoted in its entirety by 1,comprising a support 2 on which a multiplicity of parallel, periodicallyarranged diffraction structures 3 are formed. The detail in FIG. 1 showsapproximately two grating periods. The support 2 may be embodied as ametal support body, or made of quartz glass or another non-metallicmaterial with a metal coating. A metal support 2 made of aluminum isused in the exemplary embodiments described below.

The surface of each diffraction structure 3 consists of two surfacesections, each of which being inclined differently with respect to abase face 4 which is defined by the support 2.

The surface section inclined more steeply with respect to the base face4 is formed by a blaze flank 5, which interacts with incident light raysin a way which will be further described below. The blaze flank 5 isinclined with respect to the base face 4 by an angle θ of 78.7°, whichis a Littrow angle for incident light rays having a wavelength of 193.35nm.

The other surface section of the diffraction structure 3 does notdirectly receive light rays when the Littrow grating 1 is being used.This other surface section is formed by a back flank 6 extending betweenthe blaze flanks 5 of two adjacent diffraction structures 3. The blazeflank 5 and the back flank 6 of a diffraction structure 3 make an apexangle α of 90° between them. A smaller or larger apex angle may also beprovided as an alternative.

The diffraction structures 3 are provided with a transparent protectivelayer 7, which is composed of a blaze flank protective layer 8 for theblaze flank 5 and a back flank protective layer 9 for the back flank 6.

The protective layer 7 consists of MgF₂ in the embodiment shown inFIG. 1. The layer thickness C_(b) of the blaze flank protective layer 8is greater than that C_(l) of the back flank protective layer 9. Thechoice of the layer thicknesses C_(b), C_(l) has an effect on thereflection efficiency and the absorption of the Littrow grating withrespect to the incident light rays, as will be explained below.

In the following the function of the Littrow grating shown in FIG. 1will be explained.

In relation to coherent light rays with parallel incidence, which areemitted by a light source (not shown) and among which the light rays 10,11 are represented in FIG. 1 by way of example, the Littrow grating 1 isarranged so that the blaze flanks 5 are perpendicular to the incidentlight rays 10, 11. When they are reflected by the blaze flanks 5, thelight rays 10, 11 pass through the blaze flank protective layer 8.

As can be found from theoretical considerations based on electromagneticdiffraction theory, light rays which strike the Littrow grating 1 inclose proximity to a back flank 6, for example the light ray 10,interact with the back flank 6 or the back flank protective layer 9 eventhough no such interaction is found from classical geometrical optics.This interaction leads to a dependency of the absorption and thereflection efficiency of the Littrow grating not only on the layerthickness of the blaze flank protective layer 8, but also on the layerthickness of the back flank protective layer 9. This will be explainedin more detail below.

The grating period D of the Littrow grating 1 is dimensioned, with83.136 grating periods per millimeter, so that constructive interferenceis obtained in a high diffraction order for the light rays 10, 11 withthe light wavelength of 193.35 nm. The Littrow grating 1 is thusoperated as an echelle grating. Typically used diffraction orders are inthe range of the 10^(th) to 150^(th) diffraction order.

The protective layer 7 is produced using a physical vapour deposition(PVD) method, which will be briefly explained below with reference toFIG. 2.

The support 2 is held in an adjustable mount (not shown) and is exposedto approximately parallel coating beams 12, which are produced by aheatable vapour source not represented in FIG. 2. The coating beams 12make a coating angle β with the base face 4 of the support 2. At thiscoating angle β, the coating beams 12 make a blaze flank coating angle Γwith the blaze flank 5 and a back flank coating angle δ with the backflank 6. The angles Γ and δ are determined by the Littrow angle θ, thecoating angle β and the apex angle α.

In order to obtain a predetermined layer-thickness ratio C_(b)/C_(l),the adjustable mount is used to orient the support 2 with respect to thecoating beams 12 so that the blaze flank coating angle Γ and the backflank coating angle δ satisfy the following relation:sin Γ/sin δ=C _(b) /C _(l)  (1)

The support 2 is exposed until the predetermined layer thicknessesC_(b), C_(l) have been reached. As is known to the person skilled in theart, this may for example be done using comparative measurements of thelayer thicknesses of reference supports exposed at the same time, whichin the ideal case have the same alignment as the two grating flanks.

The results of calculations based on electromagnetic diffraction theorywill be explained below. They show the dependency of the reflectionefficiency and the absorption of the Littrow grating 1 on the layerthicknesses C_(b) of the blaze flank protective layer 8 and C_(l) of theback flank protective layer 9. The layer thicknesses C_(b), C_(l) areindicated as measured perpendicularly to the blaze and back flanks,respectively. The results of the calculations are represented asthree-dimensional mesh lines in FIGS. 3 to 8, in which equal reflectioncoefficients and absorptions are respectively plotted as contour linesfor clarity. The reflection and absorption values are calculated forincident light rays 10, 11 with a TM polarization, i.e. polarizedperpendicularly to the direction in which the diffraction structures 3extend.

FIG. 3 shows the reflection efficiency as a function of the layerthicknesses C_(b), C_(l) for a grating according to FIG. 1, with asupport 2 made of aluminum and a protective layer 7 made of MgF₂.

Essentially irrespective of C_(l), the Littrow grating 1 has a maximumreflection efficiency with C_(b) in the range of from 55 to 60 nm. It isonly the position of the reflection maximum, however, and not theabsolute value which is independent of C_(l) in this case. For theaforementioned range of C_(b) between 55 and 60 nm, this occurs atC_(l)=65 nm and amounts to 80%. At lower C_(l) values, the reflectionefficiency falls off rapidly for all C_(b) values. With a C_(l) value of20 nm, only a value of about 64% can be achieved as a maximum value forthe reflection efficiency. For C_(l) values greater than C_(l)=65 nm,which are not represented in FIG. 3, the reflection efficiency falls offeven faster.

FIG. 4 shows the absorption by the Littrow grating 1 for the same C_(b),C_(l) ranges as in FIG. 3.

In the figures representing the absorption, the numerical values of theabsorption stand for the fraction of light energy absorbed. For example,an absorption value of 0.09 indicates that 9% of the light energyincident on the Littrow grating 1 will be absorbed.

The absorption minimum at a value of 0.09 is obtained for a thicknessrange of C_(b) between 55 and 60 nm and for a layer thickness C_(l) of20 nm. This absorption minimum does not correspond to thereflection-efficiency maximum (cf. FIG. 3), which is achieved at adifferent C_(l) value, namely 65 nm. The absorption is in turn notminimal at the reflection-efficiency maximum, but rather has a value ofclose to 0.18.

FIGS. 5 and 6, and respectively 7 and 8, show results calculated in thesame way as FIGS. 3 and 4 for the reflection efficiency and theabsorption of two other embodiments of a Littrow grating, which differfrom the one represented in FIG. 1 by the material of the protectivelayer 7. This material is SiO₂ in the case of the Littrow grating ofFIGS. 5 and 6, and Al₂O₃ in the case of the Littrow grating of FIGS. 7and 8.

As FIG. 5 shows, in the case of the Littrow grating with a SiO₂ coating,the reflection maximum is achieved with C_(b) in the range of between 50and 55 nm and with C_(l) close to 55 nm. Here again, the reflectionefficiency has a maximum value of about 80%. Around this reflectionmaximum, similarly as in the embodiment according to FIGS. 3 and 4, thereflection efficiency has a relatively strong dependency on C_(l) and,compared with this, a weaker dependency on C_(b). The reflectionefficiency falls off quite steeply at C_(l) values larger than 55 nmand, for example, amounts to only 40% at C_(l)=60 nm almostindependently of the values represented for C_(b), whereas it falls offless strongly for C_(l) values of less than 55 nm.

FIG. 6 shows the absorption ratios for the grating with an SiO₂ coating.A minimum absorption of about 0.11 is reached with C_(b) in the range ofbetween 50 and 55 nm and with C_(l) close to 40 nm. The absorption risesfor other C_(b) values and for higher C_(l) values. Here again, as waspreviously the case in the first exemplary embodiment, the absorptionminimum is obtained at other C_(b), C_(l) values than the reflectionmaximum.

The third exemplary embodiment, which will be described below, alsoshows similar main dependencies of the reflection efficiency and theabsorption on the layer thicknesses.

FIG. 7 shows the reflection efficiencies for the third exemplaryembodiment with an Al₂O₃ protective layer. Here, the reflection maximumis obtained at C_(b)=C_(l) close to 45 nm. The reflection efficiencylikewise amounts to about 80% in this case.

FIG. 8 shows the absorption ratios in the case of the Littrow gratingwith an Al₂O₃ coating. The absorption minimum with a value of about 0.11is obtained with C_(b) close to 45 nm and C_(l) close to 30 nm.

The profiles calculated for the reflection efficiency and the absorptionhave a common feature that the optima for the reflection efficiency andthe absorption are respectively achieved at the same values of the layerthickness C_(b) of the blaze flank protective layer 8. These valuesdepend on the refractive index of the material of the protective layer,as demonstrated by the following table which reports the optimum layerthicknesses C_(b) for the various protective-layer materials: N = n + ikOptimum layer Material (193 nm) thicknesses C_(b) (nm) MgF₂ 1.44 57 SiO₂1.56 52 Al₂O₃ 1.78 + i · 0.001 45

The refractive indices N at 193 nm are also indicated here for thematerials. Since Al₂O₃ has a non-negligible absorption at 193 nm, acomplex refractive index is indicated in this case.

As comparative calculations have shown, these values for the optimumlayer thickness C_(b) on the blaze flank correspond approximately to theresults of a conventional layer-thickness calculation, as is known forhighly reflective coatings on flat supports. Besides the refractiveindex of the protective-layer material, the complex refractive index ofthe support material is also included in this calculation.

From the results of electromagnetic diffraction theory (cf. FIGS. 3 to8) or the results of the conventional layer-thickness calculation forC_(b), it is straightforward to provide a layer thickness C_(b) which isvery close to the optimum layer thickness, for a differentprotective-layer material. This quasi-optimum layer thickness C_(b) ^(n)for a new protective-layer material is found from the empirical relationC _(b) ^(n) =C _(b) n/n.  (2)where n denotes the refractive index of the protective-layer materialfor which the optimum layer thickness C_(b) is known, and n^(n) denotesthe refractive index of the new material.

Instead of a single protective layer 7 on the blaze flank 5, as was thecase in the exemplary embodiments discussed above, a multilayer systemmay also be applied to the blaze flank in another variant of the Littrowgrating. To that end, a second material with a higher refractive indexn_(H) is used in addition to a first material for the protective layerwith a refractive index n.

FIG. 9 shows an example of such a multilayer system. As is known forhighly reflective dielectric multilayer systems on flat substrates,starting from the metal layer of the support 2, the following layersequence is selected:

support 2

layer 20 with refractive index n

layer 21 with refractive index n_(H)

Depending on the desired number of layers in the multilayer system, thismay be supplemented with further layer pairs 22, 23 (by two of them inthe example of FIG. 9) with the sequence:

layer 22 with refractive index n

layer 23 with refractive index n_(H),

each with an optical thickness of λ/4.

In the example of FIG. 9, for a laser wavelength of 193 nm, the valuesof the refractive indices and the layer thicknesses of the layers 20 to23 may be as follows:

layer 20 with a refractive index of 1.44 and a layer thickness of 21 nm

layer 21 with a refractive index of 1.78 and a layer thickness of 27 nm

layers 22 with a refractive index of 1.44 and a layer thickness of 34 nm

layers 23 with a refractive index of 1.78 and a layer thickness of 27nm.

The thickness of the first layer 20 applied to the support 2 will inthis case be selected according to the result of a ray-opticallayer-thickness calculation for thin-film systems to minimize reflectionfor a dielectric coating on a flat reflective base layer, namely thesupport 2, while taking the complex refractive index of this base layerinto account. All other layers 21 to 23 of the multilayer system havethe optical thickness λ/4.

If the blaze flank is to be coated with such a multilayer system, thenthe blaze flank and the back flank will not be coated together asrepresented in FIG. 2, but instead independently of each other. In afirst method step, the multilayer system is in this case applied to theblaze flank, the Littrow grating being arranged so that the back flanklies in the coating shadow of the coating beams. When the back flank isbeing coated, account is optionally be taken of the fact that a smallproportion of the coating material may already have been deposited on itwhile the blaze flank was being coated. It is also possible to apply thefirst layer of the multilayer system together with the layer on the backflank, as was described above in connection with the single protectivelayers.

In order to be able to provide a layer thickness C_(l) to minimize theabsorption for the back flank even for other material combinations(comprising support material and protective-layer material) than thosewhich were described above, without resorting to electromagneticdiffraction theory, an approximation formula for calculating C_(l) isgiven below. To minimize the absorption by the Littrow grating, thelayer thickness C_(l) should essentially satisfy the following relation:C _(l)=λ/(4n)  (3)

Here, λ denotes the wavelength of the light rays interacting with thegrating and n denotes the refractive index of the protective-layermaterial for the back flank. The layer thickness C_(l) should not differfrom this value by more than λ/(8n).

A corresponding approximation formula can also be given for the layerthickness C_(l) to achieve maximum efficiency. In order to maximize thereflection efficiency of the Littrow grating, the layer thickness C_(l)should essentially satisfy the following relation:λ/(4n)≦C _(l)<λ/(2n)  (4)

In the event that this interval for C_(l) is too broad, the range forC_(l) may be restricted to from 0.7 to 0.9 times the upper limit of theabove formula (4).

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the described exemplaryembodiments of the present invention without departing from the spiritor scope of the invention. Thus, it is intended that the presentinvention cover all modifications and variations of this inventionconsistent with the scope of the appended claims and their equivalents.

1. An optical grating comprising a plurality of parallel diffractionstructures, which are arranged on a support defining a base face, eachcomprise a blaze flank, inclined substantially at a Littrow angle withrespect to the base face, and a back flank, and together form areflection layer which comprises a reflective base layer and atransparent protective layer connected to the base layer and coveringthe base layer, wherein the protective layer on the blaze flank has anaverage layer thickness C_(b) and the protective layer on the back flankhas an average layer thickness C_(l), the values of the average layerthicknesses C_(b) and C_(l) being selected independently of each othersuch that the absorption of coherent light by the optical grating isminimized.
 2. The optical grating of claim 1, wherein the opticalthickness of the protective layer on the back flank is approximately onequarter of the light wavelength of the coherent light.
 3. The opticalgrating of claim 1, wherein the average layer thickness C_(b) of theprotective layer on the blaze flank is selected according to the resultof a ray-optical layer thickness calculation for thin-film systems, saidcalculation being adapted to maximize the reflection for a dielectriccoating on a flat reflective base layer and taking into account thecomplex refractive index of the flat reflective base layer.
 4. Theoptical grating of claim 3, wherein the protective layer on the blazeflank is designed as a multilayer system comprising successive layers oftwo dielectrics having different refractive indices.
 5. The opticalgrating of claim 1, wherein the coherent light has a TM polarizationwhen the coherent light impinges on the grating.
 6. An optical gratingcomprising a plurality of parallel diffraction structures, which arearranged on a support defining a base face, each comprise a blaze flank,inclined substantially at a Littrow angle with respect to the base face,and a back flank, and together form a reflection layer which comprises areflective base layer and a transparent protective layer connected tothe base layer and covering the base layer, wherein the protective layeron the blaze flank has an average layer thickness C_(b) and theprotective layer on the back flank has an average layer thickness C_(l),the values of the average layer thicknesses C_(b) and C_(l) beingselected independently of each other such that the diffractionefficiency for the coherent light is maximized.
 7. The optical gratingof claim 6, wherein the layer thickness C_(l) of the protective layer onthe back flank satisfies the following relation:λ/(4n)≦C _(l)<λ/(2n) where λ denotes the wavelength of the coherentlight and n denotes the refractive index of a material from which theprotective layer is made.
 8. The optical grating of claim 6, wherein theaverage layer thickness C_(b) of the protective layer on the blaze flankis selected according to the result of a ray-optical layer thicknesscalculation for thin-film systems, said calculation being adapted tomaximize the reflection for a dielectric coating on a flat reflectivebase layer and taking into account the complex refractive index of theflat reflective base layer.
 9. The optical grating of claim 8, whereinthe protective layer on the blaze flank is designed as a multilayersystem comprising successive layers of two dielectrics having differentrefractive indices.
 10. The optical grating of claim 6, wherein thecoherent light has a TM polarization when the coherent light impinges onthe grating.
 11. The optical grating of claim 6, wherein the protectivelayer on the back flank has an average layer thickness C_(l) which islarger than the average layer thickness C_(b) of the protective layer onthe blaze flank.
 12. The optical grating of claim 6, wherein the averagelayer thickness C_(b) of the protective layer on the blaze flank and theaverage layer thickness C_(l) of the protective layer on the back flankare less than 100 nm.
 13. The optical grating of claim 6, wherein thebase layer has a metal surface.
 14. The optical grating of claim 13,wherein the metal surface is formed by a metal layer applied to thesupport.
 15. The optical grating of claim 13, wherein the metal isaluminum.
 16. The optical grating of claim 6, wherein the protectivelayer comprises a dielectric layer.
 17. The optical grating of claim 16,wherein the protective layer is made of MgF₂.
 18. The optical grating ofclaim 16, wherein the protective layer is made of SiO₂.
 19. The opticalgrating of claim 16, wherein the protective layer is made of Al₂O₃. 20.A method of manufacturing an optical grating comprising: a) producing agrating blank having a support defining a base face, a plurality ofparallel diffraction structures which each comprise a blaze flank,inclined substantially at a Littrow angle with respect to the base face,and a back flank, and which together form a reflection layer whichcomprises a reflective base layer and a transparent protective layer,and a base layer; b) defining a layer thickness C_(b) of the protectivelayer on the blaze flank and a layer thickness C_(l) of the protectivelayer on the back flank; c) determining an orientation angle β between apreferential coating direction of a coating source and the base facesuch that the ratio of thickness increases on the blaze flank and theback flank corresponds to C_(b)/C_(l); d) positioning the grating blankin the coating system at the orientation angle β determined in the stepc); and e) coating the grating blank until an average layer thickness ofthe protecting layer on the blaze flank has reached the thickness C_(b)and an average layer thickness of the protecting layer on the back flankhas reached the thickness C_(l).
 21. The method according to claim 20,wherein the grating protecting layer is coated in the step e) using aPVD method.